1. Field of the Invention
The present invention relates to process for producing silicon carbide-base complex, and more particularly to high purity silicon carbide-base complex which is useful as a jig for producing semiconductors, for example, a heat resistance jig material such as process tube, wafer boat used for heat doping operations, and a process for producing the same. On this occasion, the term "complex" comprehends composites.
2. Description of the Relevant Art
In the past, silica glass has been used as the heat resistance jig for producing semiconductors of the required high purity. The silica glass has several merits, for example, a extremely high purity jig can be produced easily by using it, and its interior can be easily observed because of its transparency.
The silica glass-made jig, however, has also several demerits, for example, it can hardly be used it for heat treatment at a temperature of 1150.degree. C. or more because of it starting to change its dimension at 1000.degree. C. due to viscous flow, and it has a short life because of changing to .alpha.-cristobalite causes it to lose transparency an to be broken.
In recent years, a complex produced by charging porous silicon carbide molding molded from a silicon carbide powder with metallic silicon, has been developed as a material which can solves above-mentioned problems instead of silica glass and, has been used as the heat resistance jig for producing semiconductors.
E. G. Acheson has been considered to discover silicon carbide in 1891. The Acheson method, in which silicon carbide is synthesized by means of a carbothermic reduction with silica rock and coke, has been known as a first industrial method for producing silicon carbide. This reaction is shown as the following formula (1): EQU SiO.sub.2 +3C.fwdarw.SiC+2CO (1)
In recent years, in the process of producing semiconductors such as LSI, the use of a heat resistance material mainly composed of silicon carbide for a reactor core tube and a wafer supporting material when silicon wafer is heat-treated in the range of above 1100.degree. C. has increased. On the other hand, the influence of a trace of impurities contained in the reactor core tube and the wafer supporting material has become a problem with high integration of LSI.
In Acheson method described above, the silicon carbide material is produced by means of mixing a silicon source such as quartz sand, crystal powder and colloidal silica with a carbon source such as coke, tar pitch and carbon black, and sintering this mixture. As a result, the silicon carbide material is contaminated by between several ten ppm and several % caused by metallic impurities contained in the silicon source and the carbon source, and also caused by the container, jig, atmosphere in the steps of mixing, molding, sintering, etc.
Further, the silicon carbide-base material is inadequate for a heat resistance material of semiconductor in case of using the contaminated silicon carbide-base material powder, even though the silicon carbide-base material is produced densely by means of a silicon impregnation method without a sintering assistant.
According to severe demands to obtain a high purity silicon carbide material, the process for producing the silicon carbide material powder has been industrialized by means of vapor phase synthesis. The silicon impregnation method, which comprised the steps of sintering the high purity silicon carbide material powder and producing a dense silicon carbide-base material without sintering assistant, has been disclosed in Japanese Patent Laid-Open Publication No. 43553/82.
In the Acheson method as described above, since quartz sand, and crystal powder and colloidal silica are used as the silicon source, and coke, tar pitch and carbon black are used as the carbon source, the silicon carbide material becomes contaminated from several ten ppm to several % by metallic impurities contained in the silicon source and the carbon source, and also by the container, the jig, the atmosphere in the steps of mixing, molding, sintering, etc. Therefore, it is difficult to synthesize a high purity silicon carbide-base material.
In addition, a method comprising the step of heating metallic silicon at a temperature at its melting point or more and impregnating the melted silicon has been used generally (Japanese Patent Laid-Open Publication Nos. 85374/76, 14914/89, 115888/89). Also, a method comprising the steps of evaporating silicon by means of induction heating of graphite molding and impregnating silicon into the silicon carbide body has been disclosed (Japanese Patent Laid-Open Publication No. 43553/82).
A method comprising the step of producing a dense silicon carbide membrane on the surface of silicon carbide material by means of vapor deposition of silicon carbide, the membrane inhibiting impurity diffusion, has been disclosed.
The silicon carbide-base material being used for producing semiconductors, has been produced by means of a purifying treatment after molding and sintering. However, not only is the material contaminated during the steps of these complex process for producing it, but it also removes impurities from only the surface because the purifying treatment is performed after sintering. As a result, in case that the silicon wafer is heat-treated with the silicon carbide-base material-made jig produced by the above-mentioned method, the jig may contaminate the silicon wafer because of diffusional release of impurities such as Fe in the material composed of silicon carbide.
Also, according to a method comprising the step of impregnating a silicon carbide body with molten or melted metallic silicon, large amount of energy is required to raise the temperature to above 2000.degree. C. and thermal deformation and cracks may be induced into the silicon carbide body by thermal shock.
According to the method which comprises producing the dense silicon carbide membrane on the surface of the silicon carbide material by means of vapor deposition of silicon carbide, and in which the membrane inhibits impurities diffusion, the silicon carbide-base material, produced by means of the above-mentioned method, induces pin-holes on its surface and cracks by mechanical or thermal shock because of a low adhering strength of the membrane.
Moreover, the silicon carbide-base material becomes from the container, the jig, and the atmosphere in the steps of storage of the silicon carbide material powder, molding the silicon carbide powder, or sintering this molding, even though the synthesized high purity silicon carbide material powder produced by means of vapor phase synthesis is used. Therefore, it is difficult to produce the extremely high purity silicon carbide-base complex for producing a semiconductor.
Only in the case of forming a dense silicon carbide thick film directly on a high purity graphite molding or a high purity silica glass-made molding, is it possible to produce a high purity silicon carbide-base complex. However, according to the above-mentioned method, it still has problems to be solved because of slow growth reaction of the silicon carbide, the expense for producing, and breaking the silicon carbide-base material due to a difference of thermal expansion coefficient between silicon carbide and graphite and silica glass as base material.
The silicon carbide powders being used for heat resistant ceramic materials have a high purity and a high degree of sintering, and have been required to be supplied as fine particles. In addition, the silicon carbide materials being used for a jig for producing semiconductors have been required to have a high purity and not to contaminate the silicon wafers.
In the past, a silicon carbide powder has been produced by a method which comprises crushing .alpha.-type silicon carbide produced by means of the Acheson method, and then passing it through a sieve (Japanese Patent Laid-Open Publication No. 84013/78).
In order to obtain a dense sintered molding using the silicon carbide powder produced by above-mentioned method, the sintering temperature required is an extremely high temperature, because the silicon carbide powder produced by above-mentioned method is .alpha.-type silicon carbide which stabilizes under high temperature, and does not accompany transformation and dislocation of crystal-type at sintering, and its sintering speed is slow. Also, this silicon carbide powder cannot be used for producing semiconductors because of some contaminant caused by the step of crushing and by material such as quartz sand and coke. In addition, the impurities which the silicon carbide powder includes causes breaking as well as a strength scattering of the sintered molding.
Contrarily, a method, in which .beta.-type silicon carbide particles are produced by means of vapor phase reaction with silicon halide, silicon carbide, etc., has been disclosed in Japanese Patent Laid-Open Publication No. 160200/75. Moreover a method in which a high purity .beta.-type silicon carbide is produced by means of pyrolysis of an organic silicon compound, has been disclosed in Japanese Patent Laid-Open Publication No, 67599/79.
However, according to the former of the above-mentioned methods, there is the possibility of contaminating the silicon carbide particles with metal halide gas which is formed in the step that hydrohalide gas corrodes tube because corrosive hydrohalide gas exists as by-product in this reaction system.
In addition, according to the latter of the above-mentioned methods, it is required to remove free carbon by means of sintering under an oxidizing atmosphere because of remaining free carbon in vapor phase as kinds of organic silicon compound, or as a condition of pyrolysis.
Further, because it is required to separate and recover the silicon carbide powder deposited from the vapor phase, the silicon carbide powder causes lowering of the yield and contamination.